The present invention relates to an improved fire sensing system in which a first or a second semiconductor switching element which will be rendered conductive by the detection output from a fire sensor is connected to at least two power-supply circuits which actuate the fire sensors, and in which noise energy induced in the system is absorbed by the first semiconductor switching element relying upon a substantial time difference which provides a time interval for the operation of the first and the second semiconductor switching elements, so that the second semiconductor switching element or the power-supply circuit including the second semiconductor switching element is maintained in a proper condition.
A fire sensor capable of producing a plurality of detection outputs has been proposed in Japanese Utility Model Laid-Open No. 24284/78, consisting, as shown in FIG. 1 of the present application, of a switching element 4 which operates in synchronism with the operation of an oscillation circuit 1, a plurality of switching elements 5a, 5b and 5c for successively changing resistors R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 connected in series with the switching element 4 from a high sensitivity toward a low sensitivity, a fire detecting switching element 9 provided in a fire detection unit 6 which produces fire detection signals proportional to the smoke concentration produced by a fire, a scanning circuit 10 which is operated by the output of the switching element 9 to successively render the switching elements 5a to 5c conductive thereby changing the sensitivity, an alarm producing switching element 11 which is operated by the output of the fire detecting switching element 9 to short-circuit lines 2a, 2b so that the impedance therebetween is decreased, and a restoration circuit 12 which resets the scanning circuit 10 to the initial state when the output of the oscillation circuit 1 becomes out of agreement with the output of the fire detecting switching element 9.
In the fire sensor shown in FIG. 1, however, if noise or the like is generated, the scanning circuit employing a shift register is operated even when the alarming switching element 11 is not rendered conductive so that the switching elements 5a and 5c are operated causing the detection sensitivity to be changed. Therefore, the fire sensor shown in FIG. 1 is not practicable unless the scanning circuit 10 is powered by a separate power supply or is isolated by using a photo-coupler in the input/output signal stage of scanning circuit 10. Therefore, the circuitry of the existing fire sensor inevitably becomes complicated. Further, a fire sensor having a plurality of sensitivities is usually intended to be interlocked to facilities for preventing and exhausting smoke in addition to providing fire alarms. According to the conventional example illustrated in FIG. 1, a receiving unit is interlocked to other fire-preventing facilities subsequent to the fire alarm, responsive to the number of signals produced by the same fire detector. Hence, in case a plurality of fire sensors connected to the same circuit produce alarms, the receiving unit is not capable of discriminating at which sensitivity such alarms are produced. There has also been proposed a system utilising two or more detection outputs as illustrated in FIG. 2, by employing, in addition to a fire sensor for producing a fire alarm, a fire sensor which detects a combustion quantity to produce an output and by energizing safety facilities such as those for preventing and exhausting smoke relying on the output of the latter fire sensor which detects the combustion quantity. According to this system, a circuit L.sub.3 is run from a given sensor, for example, from a sensor S.sub.2 among sensors S.sub.1, S.sub.2, S.sub.3, etc. connected to sensor circuits L.sub.1, L.sub.2 from a receiver R, thereby to supply an interlocked relay B.sub.1 for the receiver R. A contact b.sub.1 of the interlocked relay inserted in the circuits L.sub.4, L.sub.5 is closed to energize a response device Z for actuating equipment to prevent or exhaust smoke (to actuate the facilities for preventing and exhausting smoke) when the interlocked relay B.sub.1 is energized by the operation of the sensor S.sub.2. A relay B.sub. 2 is energized by a contact b.sub.3 which will be closed when the operation is completed, such that the interlocked state can be confirmed by the illumination of display lamp. At the same time, a contact a.sub.1 of a fire alarm relay A.sub.1 of the receiving unit R is closed by the operation of the sensor S.sub.2, whereby the sensor circuits L.sub.1, L.sub.2 are short-circuited to produce a fire alarm. There will be no problem if the sensor S.sub.2 interlocked to other facilities is energized first. However, when another sensor, for example, a sensor S.sub.1 having no relation to other facilities is energized first, the closure of the contact a.sub.1 of the fire alarm relay A.sub.1 causes the sensor circuits L.sub.1, L.sub.2 to be short-circuited so that the impedance is decreased. Consequently, the sensor S.sub.2 interlocked to other facilities is not served with sufficient power-supply voltage, causing the system to be inoperative even when smoke has infiltrated into the system's sensors.
According to a further conventional example disclosed in U.S. Pat. No. 3,909,814, a plurality of semiconductor switching elements are connected to a power-supply circuit in parallel with each other, and fire sensors are connected in parallel with the switching elements to successively render them conductive responsive to increases in the combustion quantity. According to this embodiment in which many semiconductor switching elements for controlling different alarm signals are connected to the same power supply in parallel with each other, there is no way to specify a signal line which works to absorb noise energy upon the receipt of noise.